Adjustable office seats and arm-chairs are certainly known. To allow a more comfortable seat with reference to those of lower quality of the static type, they provide, in addition to a device to adjust the height, the use of a device for controlled oscillation, usually positioned in the part immediately below the surface of the seat and integral with it. Said device can further be actuated by means of a lever, which projects and can be easily gripped and thus rotated in one direction then in the other, until the internal device does not provide the unlocking of the articulation.
Briefly, it is therefore possible to affirm that the following are known:                chairs in which the seat and the backrest are on separate bodies, the two being connected in such a way that an inclination of the backrest corresponds to a movement parallel to the ground of the seat;        chairs in which the backrest is exclusively free to oscillate;        chairs in which the lifting of the seat surface corresponds to the inclination of the backrest;        chairs in which both the seat and the backrest are individually adjustable;        finally, chairs where both the seat and the backrest, being interconnected, carry out a synchronized inclination movement.        